.container {
    margin: 20px auto;
    width: 650px;
}

.smallBox {
    position: relative;
    float: left;
    width: 300px;
    height: 300px;
    overflow: hidden;
}

.smallBox img {
    display: block;
    width: 100%;
    height: 100%;
}

.smallBox .mark {
    position: absolute;
    top: 0;
    left: 0;
    z-index: 10;
    width: 100px;
    height: 100px;
    background: rgba(255, 0, 0, .3);
    cursor: move;
}

.bigBox {
    display: none;
    position: relative;
    float: left;
    width: 350px;
    height: 350px;
    overflow: hidden;
}

.bigBox img {
    position: absolute;
    top: 0;
    left: 0;
    /*mark/small-box===big-box/大图*/
    width: 1050px;
    height: 1050px;
}